This invention relates generally to operational amplifiers, and more particularly, to an all NPN monolithically integrable output stage for an operational amplifier which exhibits an improved gain characteristic.
The majority of commercially available operational amplifiers which possess both pull-up and pull-down output capability include PNP devices as part of the output stage. For example, see U.S. Pat. No. 3,660,773. To achieve a higher frequency response, greater output swing, reduced output stage emitter-follower peaking and to simplify the integrated circuit construction, it has been found desirable to provide an output stage which incorporates only NPN transistors. Such an output circuit is shown and described in U.S. Pat. No. 4,403,200 entitled "OUTPUT STAGE FOR OPERATIONAL AMPLIFIER" and assigned to the assignee of the present invention. While the circuit shown and described exhibits satisfactory phase margin (i.e. excess phase at unity gain), the gain characteristic of the device is such that after it passes down through the unity gain frequency, the gain rises again at a higher frequency. If the device has gain at the point where the phase approaches zero, the circuit will oscillate. This tendency to oscillate at higher frequencies in the unity gain configuration renders the frequency response of the device undesirable.
To correct this situation, an improved output stage was developed wherein a Miller capacitor is coupled between the base and collector terminals of at least one of the NPN pull-down output transistor as is shown and described in co-pending application Ser. No. 494,148 filed May 13, 1983 entitled "MILLER COMPENSATION FOR AN OPERATIONAL AMPLIFIER" and now U.S. Pat. 4,513,251, and assigned to the assignee of the present invention. In this manner, the Miller capacitor compensates both the positive and negative loops of the device. However, since the positive or pull-up loop is characterized by more gain than the negative or pulldown loop, the phase performance of the negative loop is seriously degraded. It would therefore be desirable to provide compensation means which primarily affects the positive loop with only a minor effect on the negative loop thus avoiding phase problems in the negative loop.